In 2000, the American Society for Testing and Materials developed an AM
0 reference spectrum (ASTM E-490) for use by the aerospace community.
That ASTM E490 Air Mass Zero solar spectral irradiance is based on data
from satellites, space shuttle missions, high-altitude aircraft, rocket
soundings, ground-based solar telescopes, and modeled spectral
irradiance. The integrated spectral irradiance has been made to conform
to the value of the solar constant accepted by the space community;
which is 1366.1 W/m2.

In the 0.1195 to 0.41 m range, the values are averages of two different
instruments on the Upper Atmosphere Research Satellite (UARS), the Solar
Ultraviolet Spectral Irradiance Monitor (SUSIM) and the SOLar STellar
Irradiance Comparison Experiment (SOLSTICE), reported by
Woods et al.
These data were obtained in April 1993 during a period of moderate
solar activity, and were scaled by a factor of 0.96843 to match the
Neckel and Labs
data over the 0.33 to 0.41 m range.
In the 0.41 to 0.825 m range, the values are from the McMath Solar
Telescope at Kitt Peak, Arizona, as reported by
Neckel and Labs.
In the 0.825 to 4.0 m range, the values are from the high-resolution
solar atlas computed by Kurucz.
These data were smoothed to 2-and 20-nm wavelength resolution
and scaled by a factor of
1.00085 to match the Neckel and Labs
data at 0.825 m.
In the 4.0 to 1000 m range, the values are from the logarithmic
irradiance versus wavelength fits reported by
Smith and Gottlieb.
These data were scaled by a factor of 0.99437 to match the
Kurucz data at 4.0 m.
The composite spectral irradiance data were then scaled by a factor of
0.99745 to force the integrated total irradiance equal to the solar
constant.

The WMO/WRDC Wehrli Air Mass Zero (AM 0) solar spectral irradiance curve
has often been cited and used as the extraterrestrial solar spectal
irradiance distribution. This spectral distribution was constructed in
1985, based on the following references:

Wehrli cites four sources for the data used to construct the WMO
Extraterrestrial (ETR) spectral distribution.
A preliminary version of this spectral distribution is integrated into the
NREL Simple Spectral
Model SPCTRAL2; and is also the ETR used in generating the ASTM Reference
Spectra for terrestrial applications, namely E-891 and E892, which have been
combined into ASTM G-159.

There are other, nonstandard AM0 (extraterrestrial, or ETR) solar spectra that
have been derived since 1980. Several of these spectra are available as
components in the MODTRAN and SMARTS models for atmospheric spectral
transmission of sunlight.